Pain is broadly defined as an unpleasant sensory experience associated with actual or potential tissue damage, or described in terms of such damage. The interpretation of sensory pain occurs when peripheral nerve endings called nociceptors are stimulated and subsequently transmit signals through sensory neurons in the spinal cord. The signals are then transmitted to the brain, at which point the individual becomes aware of the pain.
There are a number of pain categories and classifications. For example, the experience of physiological pain can be grouped into four categories according to the source and related nociceptors.
First, cutaneous pain is caused by injury to the skin or superficial tissues. Cutaneous nociceptors terminate just below the skin, and due to the high concentration of nerve endings, produce a well-defined, localized pain of short duration. Example injuries that produce cutaneous pain include paper cuts, minor burns (e.g., first degree burns) and superficial lacerations.
Second, somatic pain originates from ligaments, tendons, bones, blood vessels, and even nerves themselves, and is detected with somatic nociceptors. The scarcity of nociceptors in these areas produces a sharp, aching, pain of longer duration than cutaneous pain and somewhat less localized. Examples include a sprained ankle or broken bones.
Third, visceral pain originates from body organs. Visceral nociceptors are located within body organs and internal cavities. Similar to somatic pain, a scarcity of nociceptors in these areas produces a pain usually more aching and of a longer duration than somatic pain. Visceral pain may be more difficult to localize. Injuries to visceral tissue may exhibit “referred” pain, where the sensation is localized to an area completely unrelated to the site of injury. Myocardial ischaemia (i.e., the loss of blood flow to a part of the heart muscle tissue) is an example of referred pain; the sensation can occur in the upper chest as a restricted feeling, or as an ache in the left shoulder, arm, or hand. Another example of referred pain is phantom limb pain. Phantom limb pain is the sensation of pain from a limb that a person no longer has or from which the person no longer receives physical signals. This phenomena—also known as deafferentation pain—is almost universally reported by amputees and quadriplegics.
Fourth, neuropathic pain (e.g., “neuralgia”) can occur as a result of injury or disease to the nerve tissue itself. The injury or disease can disrupt the ability of the sensory nerves to transmit correct information to the thalamus or cortex. Consequently, the brain interprets painful stimuli even though there is no obvious or documented physiologic cause for the pain.
Other pain classifications include acute pain and chronic pain. Acute pain is defined as short-term pain or pain with an easily identifiable cause. Acute pain indicates present damage to tissue or disease and may be “fast” and “sharp” followed by aching pain. Acute pain is centralized in one area before becoming somewhat spread out. Acute pain generally responds well to medications (e.g., morphine).
Chronic pain may be medically defined as pain that has lasted six months or longer. This constant or intermittent pain has often outlived its purpose because it does not help the body to prevent injury. It is often more difficult to treat than acute pain. Expert care is generally necessary to treat any pain that has become chronic. In addition, stronger medications are typically used for extended periods in an attempt to control the pain. This can lead to drug dependency. For example, opioids are used in some instances for prolonged periods to control chronic pain. Drug tolerance, chemical dependency, and even psychological addiction may occur.
Debilitating chronic pain affects tens of millions of people annually. Accordingly, this costs hundreds of millions of dollars in terms of medication, physical therapy, and lost production. The current methods for treating chronic pain have a limited success rate and in some cases may result in chemical dependency.
What is needed are safe, effective systems and methods for treating chronic, debilitating, nociceptive pain that deliver appropriate mechanical, electrical, or chemical stimulation directly to specific brain regions. In addition, effective methods are needed that may reduce the need for habit-forming pain-relieving drugs or minimize the need for such drugs by maximizing their effectiveness through direct application in specific brain regions.